OM-USB-1208FS User's Guide Functional Details
21
Synchronizing multiple units
You can run up to two OM-USB-1208FS units on most computers.
You can connect the SYNC pin of two OM-USB-1208FS units together in a master/slave configuration and
acquire data from the analog inputs of both devices using one clock. When the SYNC pin is configured as an
output, the internal A/D pacer clock is sent to the screw terminal. You can use this signal as a clock input to a
second USB by connecting it to the SYNC pin of the second USB.
When used as a clock input, the SYNC pin operates in one of two modes – Continuous or Gated.
In the default Continuous mode, a OM-USB-1208FS ignores the first clock pulse in order to ensure adequate
setup time. Use this mode if the unit is being paced from a continuous clock source, such as a generator.
In the Gated mode, it is assumed that the clock signal will be held off for an adequate amount of time for setup
to occur. No clock pulses are ignored. Use this mode if the OM-USB-1208FS is set up as a slave and the source
of the external clock is another USB.
The SYNC pin (pin 19) is set for pacer output by default. To set it to input requires a software command (see
Figure 13 below).
To synchronize a master OM-USB-1208FS with a slave OM-USB-1208FS and acquire data, follow the steps
below.
1. Connect the SYNC pin of the master OM-USB-1208FS to the SYNC pin of the slave OM-USB-1208FS.
2. Run InstaCal.
3. From the PC Board List on the InstaCal main form, double-click on the OM-USB-1208FS you want to use
as a slave. The Board Configuration dialog opens.
4. Select Gated from the Ext. Clock Type drop-down list.
5. Set the Universal Library EXTCLOCK option with cbAInScan()/AInScan for the slave OM-USB-1208FS
to enable pacing from the master USB device.
This InstaCal option does not affect internally paced acquisition. It only affects scans that use the EXTCLOCK
option.
An example of a master/slave configuration is shown below.
Figure 13. Configuring for synchronous data acquisition
When you are operating one OM-USB-1208FS, do not set the EXTCLOCK option unless you are using an
external clock for A/D pacing.